In many electronic apparatus, such as multimeters, externally generated voltages are applied to the apparatus for processing or analysis thereof. In the case of a multimeter, voltages of various magnitudes are coupled to the meter input for measurement. Typically, if the input voltage exceeds the potential of the power supply, the input voltage is clamped to the most positive and the most negative power supply rails in the apparatus. However, in battery powered apparatus, such as multimeters, clamping excessive voltages to the supply rails can be undesirable and even dangerous. In particular, if the multimeter is implemented, in whole or part, as an integrated circuit, the voltage tolerance is very low, on the order of less than 10 volts. Excessive voltages can destroy integrated circuit elements, requiring repair or, often, complete replacement.
FIG. 1 illustrates a prior art overvoltage protection circuit 10A for use with a battery-powered apparatus. In circuit 10A, a diode 18 is coupled between input terminal 14 and positive power supply 22. A diode 20 is similarly coupled between input terminal 14 and negative power supply 24. An input voltage, V.sub.IN, from an external voltage source 12, is coupled to diodes 18 and 20 through input terminal 14 and input resistor 16. When V.sub.IN is greater than the positive power supply voltage, V.sub.DD, by at least the threshold voltage of diode 18, diode 18 begins to conduct current to the positive power supply 22, effectively charging the power supply. Similarly, when V.sub.IN is less than the negative power supply voltage, V.sub.SS, by at least the threshold voltage of diode 20, diode 20 begins to conduct current from negative power supply 24, effectively charging the negative power supply. For an electronic apparatus powered by non-alkaline batteries, such a result is acceptable. However, if the apparatus is powered by alkaline batteries, even a minute charging current, typically hundreds of microamps, will cause the alkaline batteries to generate hydrogen gas. The formation of such hydrogen gas can be very dangerous and creates both a fire and explosion hazard.
In order to overcome this difficulty, prior art circuits have used a pair of Zener diodes connected in series with reverse polarity as shown in circuit 10B of FIG. 1B. In circuit 10B, an input voltage, V.sub.IN, from an external voltage source 12, is coupled to an electrical device through input terminal 14 and input resistor 16 in a manner similar to that shown in FIG. 1A. Zener diodes 26 and 28 are connected from input resistor 16 to ground. If a positive input voltage exceeds the reverse breakdown voltage or Zener voltage of Zener diode 26, this diode begins reverse conduction. In this configuration, Zener diode 28 is forward-biased and so the positive input voltage is clamped to ground and cannot exceed the Zener voltage of diode 26.
In a similar manner, if a negative input voltage exceeds the Zener voltage of Zener diode 28, this diode begins reverse conduction. In this configuration, Zener diode 26 is forward-biased and so the negative input voltage is clamped to ground and cannot exceed the Zener voltage of diode 28.
This configuration has the advantage that the current which is generated as a result of an overvoltage condition is shunted to ground rather than the power supply. The circuit thus avoids the battery charging problems associated with the previous prior art circuit. However, when the double Zener diode circuit is used with high-impedance circuits such as CMOS circuits, the Zener clamp circuit can heavily load the input. This loading is the result of the relatively high reverse leakage current normally found in Zener diodes. This high reverse leakage current appears as a stray impedance connected across the input to ground. In the case of a multimeter circuit, this stray impedance appears as a shunt resistance and can cause errors in the readings taken with the meter.
Accordingly, a need exists for a way to safely protect an electronic apparatus from excessive voltages applied to its inputs.
It is, therefore, an object of the present invention to provide an overvoltage protection circuit which can safely shunt excessive input voltages.
Another object of the present invention is to provide an overvoltage protection circuit suitable for use with battery-powered electronic apparatus.
Yet another object of the present invention is to provide an overvoltage protection circuit which shunts current produced by an overvoltage condition to ground.
A further object of the present invention is to provide an overvoltage protection circuit which can be used with high-impedance circuitry.
Still a further object of the present invention is to provide an overvoltage protection circuit which has low leakage when in the non-conducting state.